Avoided unnecessary recreation of RelatedObjects

Refs #19399. Thanks to Track alias KJ for the patch.
Removed models/related.py BoundRelatedObject

The class wasn't used anywhere except in RelatedObject.bind(), which
wasn't used anywhere. The class had one method defined as
NotImplemented, yet the class wasn't subclassed anywhere. In short, the
class was dead code.
Fixed #19632 -- Bug in code sample.

Thanks grossmanandy at bfusa com and Simon Charette.
Fixed #19628 - Noted that app for custom user model must be in INSTALLED_APPS

Thanks dpravdin and Jordan Messina.
Fixed #19606 -- Adjusted cx_Oracle unicode detection.
Refactored makemessages command
Fix in makemessages refactoring plus UI tweaks.
Fixed #17813 -- Added a .earliest() method to QuerySet

Thanks a lot to everybody participating in developing this feature.
The patch was developed by multiple people, at least Trac aliases
tonnzor, jimmysong, Fandekasp and slurms.

Stylistic changes added by committer.
Added missing versionadded 1.6 to docs of earliest()

Refs #17813
Cleaned up testing  models.py added for earliest()

The main cleanup was removal of non-necessary __unicode__ method. The
tests didn't break on py3 as the string representation was never used
in the tests.

Refs #17813. Thanks to Simon Charette for spotting this issue.
Fixed #19637 -- Ensured AdminEmailHandler fails silently

Thanks lsaffre for the report. Refs #19325.
Fixed #18051 -- Allowed admin fieldsets to contain lists

Thanks Ricardo di Virgilio for the report, Mateus Gondim for the
patch and Nick Sandford for the review.
Fixed error introduced when testing patch for 013db6ba85

Shame on me.
Simplified a i18n test.
Fixed #19596 -- Use `_default_manager` instead of `objects` in the auth app.

This is needed to support custom user models which don't define a manager
named `objects`.
Fixed #19523 -- Improved performance of Django's bash completion

Previous version took about 150ms to source, even on a warm cache,
primarily because it forks+execs /usr/bin/basename 44 times.
This patch makes it faster by a factor of 5 (and I imagine that a little
more thought would reduce the time to effectively zero).
Fixed #19640 - Added inlineformset_factory to reference docs.

Thanks wim@ for the suggestion.
Fixed #19477 - Documented generic_inlineformset_factory

Thanks epicserve for the suggestion.
Fixed #19633 - Discouraged use of gunicorn's Django integration.
Remove dup_select_related method.

This undocumented method was used in an old version of the admin, is
totally untested and hails from 2008. Although it's listed in the
"public methods" section, as it's not documented or used I don't think
it needs a deprecation path.

If we think it's useful I'll write some tests/docs for it instead...